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Versions: (draft-krishnan-6man-overlap-fragment) 00 01 02 03 RFC 5722

IPv6 Working Group                                           S. Krishnan
Internet-Draft                                                  Ericsson
Updates: 2460 (if approved)                           September 22, 2008
Intended status: Standards Track
Expires: March 26, 2009


                 Handling of overlapping IPv6 fragments
                  draft-ietf-6man-overlap-fragment-00

Status of this Memo

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   This Internet-Draft will expire on March 26, 2009.

Abstract

   The fragmentation and reassembly algorithm specified in the base IPv6
   specification allows fragments to overlap.  This document
   demonstrates the security issues with allowing overlapping fragments
   and updates the IPv6 specification to explicitly forbid overlapping
   fragments.









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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Conventions used in this document . . . . . . . . . . . . . 3
   2.  Overlapping fragments . . . . . . . . . . . . . . . . . . . . . 3
   3.  The attack  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Recommendation  . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7







































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1.  Introduction

   Fragmentation is used in IPv6 when the IPv6 packet will not fit
   inside the path MTU to its destination.  When fragmentation is
   performed an IPv6 node uses a fragment header as specified in section
   4.5 of the IPv6 base specification [RFC2460] to break down the
   datagram into smaller fragments that will fit in the path MTU.  The
   destination node receives these fragments and reassembles them.  The
   algorithm specified for fragmentation in [RFC2460] does not prevent
   the fragments from overlapping, and this can lead to some security
   issues with firewalls [RFC4942].  This document explores the issues
   that can be caused by overlapping fragments.

1.1.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


2.  Overlapping fragments

   Commonly used firewalls use the algorithm specified in [RFC1858] to
   weed out malicious packets that try to overwrite parts of the
   transport layer header to bypass inbound connection checks.
   [RFC1858] prevents an overlapping fragment attack on an upper layer
   protocol (in this case TCP) by recommending that packets with
   fragment offset 1 be dropped.  While this works well for IPv4
   fragments, it will not work for IPv6 fragments.  This is because the
   fragmentable part of the IPv6 packet can contain extension headers
   before the TCP header, making this check less effective.


3.  The attack

   This attack describes how a malicious node can bypass a firewall
   using overlapping fragments.  Consider a sufficiently large IPv6
   packet that needs to be fragmented.


   +------------------+--------------------//-----------------------+
   |  Unfragmentable  |                 Fragmentable                |
   |       Part       |                     Part                    |
   +------------------+--------------------//-----------------------+

                        Figure 1: Large IPv6 packet

   This packet is split into several fragments by the sender so that the



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   packet can fit inside the path MTU.  Let's say the packet is split
   into two fragments.

   +------------------+--------+--------------+
   |  Unfragmentable  |Fragment|    first     |
   |       Part       | Header |   fragment   |
   +------------------+--------+--------------+

   +------------------+--------+--------------+
   |  Unfragmentable  |Fragment|    second    |
   |       Part       | Header |   fragment   |
   +------------------+--------+--------------+

                     Figure 2: Fragmented IPv6 packet

   Consider the first fragment.  Let's say it contains a destination
   options header (DOH) 80 octets long and is followed by a TCP header.

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH
 |NextHdr=DOH(60)|   Reserved    |   FragmentOffset = 0    |Res|1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Identification=aaaabbbb                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==DOH
 |NextHdr=TCP(6) | HdrExtLen = 9 |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
 |                                                               |
 .                                                               .
 .                            Options                            .
 .                                                               .
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP
 |        Source Port            |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Sequence Number                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Acknowledgment Number                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Offset| Reserved  |U|A|P|R|S|F|           Window              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: First Fragment

   The TCP header has the following values of the flags S(YN)=0 and
   A(CK)=1.  This makes an inspecting stateful firewall think that it is
   a response packet for a connection request initiated from the trusted
   side of the firewall.  Hence it will allow the fragment to pass.  It
   will also let the following fragments with the same Fragment
   Identification value in the fragment header to pass through.



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   A malicious node can form a second fragment with a TCP header that
   reverses the flags and sets S(YN)=1 and A(CK)=0.  This would change
   the packet on the receiving end to consider the packet as a
   connection request instead of a response.  By doing this the
   malicious node has bypassed the firewall's access control to initiate
   a connection request to a node protected by a firewall.

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH
 |NextHdr=DOH(60)|   Reserved    |   FragmentOffset = 10   |Res|0|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Identification=aaaabbbb                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP
 |        Source Port            |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Sequence Number                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Acknowledgment Number                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Offset| Reserved  |U|A|P|R|S|F|           Window              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 4: Second Fragment

   Note that this attack is much more serious in IPv6 than in IPv4.  In
   IPv4 the overlapping part of the TCP header did not include the
   source and destination ports.  In IPv6 the attack can easily work to
   replace destination ports with an overlapping fragment.


4.  Recommendation

   IPv6 nodes transmitting datagrams that need to be fragmented MUST NOT
   create overlapping fragments.  IPv6 nodes that receive a fragment
   that overlaps with a previously received fragment MUST cease the
   reassembly process and MUST ignore further fragments with the same
   IPv6 Source Address, IPv6 Destination Address and Fragment
   Identification.  It MUST also discard the previously received
   fragments with the same IPv6 Source Address, IPv6 Destination Address
   and Fragment Identification.


5.  Security Considerations

   This document discusses an attack that can be used to bypass IPv6
   firewalls using overlapping fragments.  It recommends disallowing
   overlapping fragments in order to prevent this attack.





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6.  IANA Considerations

   This document does not require any action from the IANA.


7.  Normative References

   [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
              Considerations for IP Fragment Filtering", RFC 1858,
              October 1995.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942,
              September 2007.


Author's Address

   Suresh Krishnan
   Ericsson
   8400 Blvd Decarie
   Town of Mount Royal, Quebec
   Canada

   Email: suresh.krishnan@ericsson.com




















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